Right. however your question was about "mi, tu, su" right? and not "se"

ellas = them = their = su Correcto?

Nope ... I still don't get it. If I want to say "raise your hand" to a student, I say "levanta la mano", right? There's no "se" in it, yet I'm still using "la" and not "tu". I still don't know when to use the possessive and when to use the definite article.............